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The normal bacterial microflora of 4 commercially prepared salads (coleslaw, crab salad, carrot salad, and potato salad) and 3 vegetables (green pepper, onion, and cabbage) were evaluated. Twenty-eight species of bacteria, including potential pathogens, were isolated. The foods were artificially inoculated with an avirulent mutant strain of Shigella flexneri 5 (pHS1059) to develop a method for the rapid detection of Shigella spp. Bacteria were separated from insoluble and particulate salad ingredients by filtration through shark skin filter paper and by low speed centrifugation. S. flexneri survived at 4°C in all salads for at least 11 days and up to 20 days in crab salad. The polymerase chain reaction (PCR), using primers for amplification of a 118-base pair DNA fragment from the virulence-associated spa region, present in all Shigella spp., was used to detect S. flexneri in filtrates from salads inoculated with S. flexneri 5 (pHS1059). DNA was amplified from all of the artificially contaminated salads and vegetables except green pepper. After 3–5 days of storage, the PCR also amplified S. flexneri DNA from salads that had been enriched with nutrients to increase the number of bacteria. Green peppers contained a PCR inhibitory substance that was attenuated by dilution and enrichment before the PCR. No amplification of DNA was observed in foods to which S. flexneri had not been added.

Survival and Detection of Shigella flexneri in Vegetables and Commercially Prepared Salads